Easy vacuum bottle question (for someone)

I recently bought an 87 xj limited 4.0. I'm kinda going through it from one end to the other, trying to get everything working as it should. On the pass side fender I found a vacuum line tucked into the corner under the heater blower motor with a screw in it for a plug. It seems to originate at the vacuum bottle, but I can't figure out or find any info about what it is supposed to do. The vacuum bottle has a large hose coming from the manifold (source), a small line leaves the v. bottle and passes through the fire wall beside the heater core (I suppose the source for the heat vents etc.) and then the mistery vac line leaves the vac bottle and was plugged. Does anybody know what this might have been?:confused1 Thanks in advance.

can you take a picture of it or find a picture somewhere on the web and point to it? The Renix 4.0 is a spaghetti buffet of vacuum lines.

I'll try to later. One thread mentioned an "electrial resistor looking sensor" mounted on the pass-side fender. What the sensor sensed was not mentioned. When I get home I will dig a little deeper, and look in the haynes book again. Thanks

So are you saying that its a third line coming from the vacuum bottle to this screw cap?

IIRC, the vacuum was used for HVAC and cruise control-- I'd look into the cruise if you've got it.